The present invention is directed to improvements in planetary transmissions. More particularly, the present invention is directed to planetary gear transmissions having multiple planetary gear sets employing helical cut gears for providing power sharing amongst the planetary gear sets.
Planetary gear trains have the advantage over pinion type gearing by permitting higher power densities, large gear ratios, and concentric power input and output. Increased power requirements in planetary gear trains are usually accommodated by increasing the diameter and width of the gears. If there are restrictions on the diametrical size of the gear train, increases in power can be met only by increasing the width of the gears or upping the material and machining specifications. There are practical limits to both these approaches.
An apparent solution for increasing power capacity within a limited diametric size is to add more gear sets to the train so that power is shared between more than one gear set. This results in a lesser load for each gear set but a higher total power capacity. However, there are severe practical problems with such a solution. This solution requires nearly perfect power sharing among the several gear sets. Such perfect power sharing among the gear sets would require manufacturing tolerances for the gears which are not practical for the vast majority of commercial applications. Practical manufacturing tolerances for the gears would result in uneven power sharing. Such uneven power sharing or uneven loading results in one set of gears being loaded more heavily than its designed for. This results in excessive wear and/or premature failure.
Gear transmissions having pairs of helical gears mounted on a drive shaft for engagement with respective pairs of helical gears mounted on a driven shaft are disclosed in copending U.S. patent application Ser. No. 09/167,760 filed Oct. 7, 1998 entitled Improvements In Power Sharing Gear Sets. The disclosures, including the disclosures of the specification and drawings, of prior U.S. patent application Ser. No. 09/167,760 filed Oct. 7, 1998, are hereby expressly incorporated by reference into this present application. The use of paired helical gears in multi-speed automotive transmissions is disclosed in U.S. patent application Ser. No. 09/187,905 filed Nov. 6, 1998 entitled Multi-Speed Automotive Transmission Using Paired Helical Gearing. The disclosures, including the disclosures of the specification and drawings, of prior U.S. patent application Ser. No. 09/187,905 filed Nov. 6, 1998 are hereby expressly incorporated by reference into this present application.
It is an object of the present invention to provide a planetary transmission providing a balanced load or balanced power sharing between two or more planetary gear sets.
It is also an object of the present invention to provide a planetary transmission which is compact in diametrical size but has high power capacity.
It is a further object of the present invention to provide a planetary transmission using helical gears for providing a balanced load or balanced power sharing between two or more planetary gear sets.
These and other objects of the present invention will become apparent from the following description and claims read in conjunction with the drawings.
The present invention is directed to a planetary gear transmission having multiple planetary gear sets employing helical cut gears.
Each planetary gear set in the gear train comprises a sun gear mounted on a sun shaft with the sun gear engaging a plurality of planet gears mounted in a planet gear carrier with the plurality of planet gears engaging a ring gear mounted on the transmission housing. The planet gear carrier may be the drive member with the sun shaft being the driven member. Conversely, the sun shaft may be the drive member with the planet gear carrier being the driven member.
The planetary gear transmission of the present invention employs helical cut gears for the sun gears, ring gears, and planet gears of the planetary gear sets to obtain practical balanced power sharing and load between two or more planetary gear sets in the gear train.
Helical gears, due to the helical angle of the gear cut, experience axial thrust when loaded. The magnitude of this axial thrust is directly proportional to the torque load on the gear. Power sharing and balanced load between planetary gear sets of the planetary gear transmission of the present invention is achieved by employing this axial thrust reaction and the resulting axial movement of sun gears and/or ring gears of adjacent planetary gear sets wherein sun gears and/or ring gears of adjacent planetary gear sets are mounted for axial movement with respect to the planetary transmission housing. If one of the planetary gear sets is more heavily loaded than the others, the axial thrust on the helical sun gear and helical ring gear of that set is not balanced with the axial thrust loads on the other planetary gear sets. The sun gear and/or the ring gear that is more heavily loaded, and thus experiencing a greater axial thrust load, moves axially in response to this thrust imbalance so as to achieve equal load sharing between planetary gear sets and no axial thrust imbalances.
The planet gears do not move axially during the operation of the planetary gear transmission. The axial thrust on a given planet gear due to interaction with a respective sun gear is equal and opposite to the axial thrust due to interaction with a respective ring gear. Therefore, the axial forces acting on a planet gear are equal and opposite resulting in no tendency for planet gears to move axially.